https://en.m.wikipedia.org/wiki/Headlight_flashing
Read the urban legend section:
"Beginning in the early 1980s, a widespread rumor regarding flashing headlights was spread mainly through fax, and later on the internet. The rumor stated that various gangs across the United States carry out an initiation wherein the initiate drives around at night with his headlights off. Whichever driver flashes his headlamps in response to the unlit car becomes the target; to complete the initiation, the prospective gang member must hunt down and shoot, kill, assault, or rape the target.[65] The story was widely spread by many government organizations, including the New Mexico State Police.[6] This rumor has been proven an urban legend.[66]
The story originated in Montana in the early 1980s, where it was rumored that the Hells Angels bike gang was initiating recruits in this way. By 1984, the story had spread to Eugene, Oregon, where it had morphed into a story of Latino and black gangs targeting whites.[65] In August 1993, the story once again appeared, this time spread through fax and email forwarding. Warning of a "blood initiation weekend" on 25 and 26 September, the rumor this time led some police departments to issue warnings after having received the fake ones.[65] In February 1994, a resident of Massillon, Ohio revived the rumor by issuing flyers which claimed that killings would take place at Westfield Belden Village. After a night of sending faxes to local businesses, the person was arrested for inducing panic.[65]
The rumor once again spread in October 1998, when a new fax, this time claiming to originate with a Drug Abuse Resistance Education (DARE) officer in Texas. The rumor spread further when officials in the San Diego government circulated the fax among city agencies; this version of the fax, though quickly dismissed within city government when it was found that the Sheriff's office had no real connection to it, now appeared to be a legitimate government-issued document.[65] Also in the fall of 1998, the Sheriff's office of Nassau County, Florida sent a warning about such gang initiation to the county fire department, who subsequently spread the fax to all county agencies. Police dispatcher Ann Johnson had thought the message urgent enough to send, but had not bothered to check its legitimacy.[65]"